JToolBar 放回 GridBagLayout 时出现 IllegalArgumentException

为什么这段代码会抛出一个IllegalArgumentException当工具栏被拖离 GUI，然后关闭（将其返回到 GUI）时？

我可以理解为什么在没有约束的情况下添加组件可能是不合适的，但在这种情况下，工具栏最初添加到面板（使用GridBagLayout）没有约束就不会导致这样的问题。为什么第二次及以后添加时会出现这种情况？

代码改编自这个答案 https://stackoverflow.com/a/21557361/418556，但是两个代码都显示相同的问题。

Code

import java.awt.*;
import javax.swing.*;

public class GridBagToolBarOddity {

    public static void main(String[] args) {
        Runnable r = new Runnable() {

            @Override
            public void run() {
                JPanel gui = new JPanel(new GridBagLayout());
                JToolBar tb = new JToolBar();
                tb.add(new JLabel("Drag me off, then drop me back!"));
                gui.add(tb);

                gui.setPreferredSize(new Dimension(300, 100));
                gui.setBackground(Color.RED);

                JFrame f = new JFrame("Demo");
                f.add(gui);
                f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                f.setLocationByPlatform(true);

                f.pack();
                f.setVisible(true);
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

例外

Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: 
    cannot add to layout: constraints must be a GridBagConstraint
        at java.awt.GridBagLayout.addLayoutComponent(GridBagLayout.java:702)
        at java.awt.Container.addImpl(Container.java:1120)
        at java.awt.Container.add(Container.java:966)
        at javax.swing.plaf.basic.BasicToolBarUI$FrameListener.windowClosing(BasicToolBarUI.java:1265)
        at java.awt.Window.processWindowEvent(Window.java:2051)
        at javax.swing.JDialog.processWindowEvent(JDialog.java:681)
        at java.awt.Window.processEvent(Window.java:2009)
        at java.awt.Component.dispatchEventImpl(Component.java:4861)
        at java.awt.Container.dispatchEventImpl(Container.java:2287)
        at java.awt.Window.dispatchEventImpl(Window.java:2719)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
        at java.awt.EventQueue.access$200(EventQueue.java:103)
        at java.awt.EventQueue$3.run(EventQueue.java:694)
        at java.awt.EventQueue$3.run(EventQueue.java:692)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
        at java.awt.EventQueue$4.run(EventQueue.java:708)
        at java.awt.EventQueue$4.run(EventQueue.java:706)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

这是 BasicToolBarUI 源代码的片段

    public void windowClosing(WindowEvent w) {
        if (toolBar.isFloatable()) {
            if (dragWindow != null)
                dragWindow.setVisible(false);
            floating = false;
            if (floatingToolBar == null)
                floatingToolBar = createFloatingWindow(toolBar);
            if (floatingToolBar instanceof Window) ((Window)floatingToolBar).setVisible(false);
            floatingToolBar.getContentPane().remove(toolBar);
            String constraint = constraintBeforeFloating;
            if (toolBar.getOrientation() == JToolBar.HORIZONTAL) {
                if (constraint == "West" || constraint == "East") {
                    constraint = "North";
                }
            } else {
                if (constraint == "North" || constraint == "South") {
                    constraint = "West";
                }
            }
            if (dockingSource == null)
                dockingSource = toolBar.getParent();
            if (propertyListener != null)
                UIManager.removePropertyChangeListener(propertyListener);
            dockingSource.add(toolBar, constraint);

正如您所看到的，它尝试传递constraintBeforeFloating String 作为约束，但GridBagLayout 需要GridBagConstraints。

constraintBeforeFloating = calculateConstraint();

and

private String calculateConstraint() {
    String constraint = null;
    LayoutManager lm = dockingSource.getLayout();
    if (lm instanceof BorderLayout) {
        constraint = (String)((BorderLayout)lm).getConstraints(toolBar);
    }
    return (constraint != null) ? constraint : constraintBeforeFloating;
}

因此，当浮动关闭时，UI 尝试将当前约束（字符串）传递给 GridBagLayout 并失败。